Thanks, Varti! This helped me get mobile data at least partially (mostly?) working.
In case anyone else is trying this, here is what I did:
1. Downloaded and copied "serviceproviders.xml" to /usr/share/mobile-broadband-provider-info/serviceproviders.xml following instructions at
https://github.com/gemian/gemini-keyboard-a...DebuggingMobileI don't know if this was necessary. I has to create this directory and copy the file from the command line as root; I haven't figured out how to run PCManFM as root on the Gemini.
2. Set APN:
cd /usr/share/ofono/scripts
./set-context-property 0 AccessPointName fast.t-mobile.com
3. Activate this profile:
./activate-context 1
I haven't found any documentation of the ofono scripts -- I had to guess at which were used to set which parameters.
I was able to set the MMS service center with:
./set-mms-details none http://mms.msg.eng.t-mobile.com/wapenc
This now shows up with "./list-contexts", but MMS sending and receiving still don't work. This is not critical for me, most of the time, but is annoying, especially because some (all?) iPhones seem to send all text messages by MMS not MMS, even if the messages are straight text without attachments. So I don;t receive text messages from some contacts' iPhones.
I now have mobile data working in Debian, at least on my home network (T-Moble USA). I'll be travelling aborad next month, and will find out if roaming data (critical for me) works.
Strangely, the "Mobile Data" tab in Connman still has both the "Mobile Data" and "Roaming Data" boxes under "Mobile Data" un-checked, even while connected. ("Mobile Data" *is* now checked in the top "Cellular" box in Connman.)
The Gemini connects automagically to cellular data if I reboot into Debian, or if reboot into Android and than reboot back into Debian.
I don't know how to set which services (e.g. Wi-Fi and/or celluar data) start at boot, or how to prioritize which connection is used. But on my cellular data plan, this isn't usually an issue -- I have unlimited cellular data, even while roaming in most countries, so I don't usually need to switch to, or prioritize, Wi-Fi.
Meaanwhile, I got a follow-up message form Planetcom support saying that they don't know how to enable cellular data in Linux, and don't support Linux on the Gemini.